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You must be looking at N09_AVH13C1 --NOAA-09 AVHRR NDVI product. Yes, that is only available for 1985 to 1988 period. However, the Long-term Data Record (LTDR) consists of products from AVHRR instruments on board various NOAA satellites which range from N07 (for NOAA-07 that was launched June 1981) through N19 (for NOAA-19 that was launched February 2009) and have specific period of coverage. So, to get AVHRR NDVI data from 1981 you need look at N07_AVH13C1 product which starts from 1981-06-24.
For more information visit product page at: https://ladsweb.modaps.eosdis.nasa.gov/ ... ions/ltdr/